The road from Earth to Mars and back contains no milestones or signposts. Its maps are represented with abstruse mathematical jargon about 'gravity fields of influence,' 'delta-V' and 'hyperbolic excess velocity.' Nevertheless, celestial navigation, even under unearthly rules, is still humanly conceivable. The routes were being mapped out in theory long before they were attainable in practice. The question facing analysts now is not which is the way to go, but rather what factors of propulsion and the duration of the mission are most important in choosing between dozens of alternate routes.

The common wisdom about the long voyage up and out from Earth to Mars is that boredom and restlessness will be a major concern. A well-known space reporter wrote in 'Spaceflight' in 1971 that, "Boredom and zero gravity will be the crew's biggest enemies. Boredom, which will exist only until planetfall, will be alleviated by making extensive tape libraries available to the crew." Out of the spaceship's windows, no visible sign of the vehicle's headlong progress (at more than 100,000 feet per second) would be visible. The only movement on the celestial sphere would be the week-by-week crawl of the brighter planets. The motion is compounded by the spaceship's own orbital track and the changing parallax of shifting positions.

The most noticeable and distressing symptom of the widening separation from Earth would be the growing round-trip radio communication lag, a delay which would mark the advent of an isolation hitherto unknown to space voyagers, as human conversation with colleagues and family members back earthside became impossible. Such psychological pressures, gnawing at supposedly idle minds drifting between the worlds, could well seriously diminish the mental health and alertness of the voyagers. At least, that's what numerous commentators have alleged.

Far from it. If recent spaceflight experience is any evidence, the Mars-bound crew will be overworked and constantly challenged. Far from brooding over the view of an unchanging field of stars, the voyagers might not have time to look out the windows for days on end. Far from sitting by the radio hungry for voices from Earth, the astronauts will probably come to see radio communications as an unwanted interruption of their busy schedules. There will be plenty of things to do in those eight to ten months en route.

The first order of business on the way outwards, once the whole spacecraft has been thoroughly checked out and all systems are exercised, will be to prepare for an immediate return to Earth, but only to be carried out in the event of a major spacecraft or crew emergency.

For the first several months, as the spaceship drifts slightly ahead of and ever further out from Earth, the crew has the ability to use onboard propulsion to turn back and get safely to Earth within a few weeks at most. But after about a third of the way out to Mars, the delta-V required for this abort maneuver has grown to exceed the capabilities of their vehicle's engines, so the expedition is committed to at least a Mars fly-by and a long loop around the sun.

Other mission abort modes can be formulated. The out-bound crew will spend a lot of time studying them, practicing them, and carrying out the preparations needed to keep such options open on the shortest notice. For example, a decision to cancel the entire Mars orbital/landing phase of the mission would require a major course change at Mars fly-by (probably using the Mars Entry Module's descent and ascent propulsion systems, unless that system's breakdown was the original cause of the wave-off) and again halfway back to Earth. But the necessary emergency return trajectories can be computed and would already be loaded into the navigation system pre-mission.

But that's only if things go wrong. Even when everything is running smoothly on the outbound leg, there are several distinct activities to fill the too-few days between Earth and Mars. Keeping alive and keeping the spaceship operating would certainly be significant activity, with frequent checkout runs and diagnostic inspections, but added to all of that would be the major tasks of study and practice for the Mars-side mission events. Scheduling lessons derived from all previous manned space programs will be needed to fit everything that needs to be done into the scant time available.

Probably the single most important factor in the success of the American manned space program over the decades has been the extent of crew control built into the spacecraft. The down side is the consequently required exhaustive training of the flight crews and ground crews (at Houston Mission Control and elsewhere).

On numerous occasions, hardware problems have been overcome by flexibility, ingenuity and on-site alteration of preplanned sequences. Life-threatening failures have been successfully finessed by well-trained, alert personnel in space and on Earth.

To reach this level of expertise, crewmembers work long hours on training mockups, practicing and continuously improving the procedures that have been written for nominal and emergency developments. The most sophisticated training is in the spacecraft simulators, which are hooked up to powerful computers that read all crew commands, then calculate what should be the consequence of such commands were the spacecraft really in flight. Finally the computers drive the cockpit displays and generate artificial 'views' out of the windows to show what the real effects would have made them show.

The heart of this process of training is the 'integrated simulation,' or 'sim' for short. A crew sits in their simulator, while the simulation computers feed their data into Mission Control computers, just as the data from the real spacecraft would be fed. Dozens of specialists monitor the health and happiness of the computer's imaginary spacecraft. These are the 'flight controllers,' in a specialized hierarchy under the 'flight director' who has ultimate authority in the mission. A third group of training specialists observe (but are not observed by) the crew and the flight controllers; they deliberately introduce certain malfunctions into the simulator computer's concept of the state of the imaginary spacecraft.

These malfunctions then affect the data output to the crew and to Mission Control, who must in turn react to the failures, then diagnose them quickly and accurately. The teams must then repair them, negate them, accommodate them, or decide to ignore them since false indications are an authentic class of hardware failure. Lastly, if there is no other choice, they abort the mission soon enough to save the lives of the crew. If you are too cautious, you might be tricked into aborting a repairable mission. If you are too bold, the astronauts 'die' before they can reach safety and you buy your colleagues a round at the 'Outpost Tavern' later.

These training specialists have a crucial and unsung role in the success of actual missions, even beyond their contribution to honing the participants to a razor's edge of sharpness. They also seek to find the most subtle and damaging failures or combinations of failures. They must therefore be first-rate engineering systems analysts in their own right, with a touch of the sadist, the conjurer and the clairvoyant thrown in. Many of these experts themselves become astronauts.

Vulnerabilities which they uncover in the make-believe world of 'sims' are repaired in reality by changes in hardware, software, or procedures. It's a learning process that continues right up until the actual space mission.

Such activity has worked for manned spaceflight in the past, and worked very well. The Mars mission will need this very same type of service, but it will have to be long-distance.

All of the exquisite training computers and staff now located at the Johnson Space Center in Houston will still be available, but the minutes-long radio round-trip time from Earth to the expeditionary spacecraft will add a new dimension to the complexity of the crew training problem.

Most definitely, high fidelity training will be needed on the flight. The crucial portions of the mission, Mars orbit insertion (call it 'MOI') and the landing itself, will take place almost a year after the last chance for earthside training. There's no way the crewmen can store away those reflexes and learned procedures to remain dormant in their minds for such a period. At the very least, refresher training must be scheduled, but if so, the outbound voyage then might as well include the major portion of the key training for the at-Mars activities anyway.

Therefore, the Mars spacecraft must be designed from the beginning with in-flight training in mind. All flight controls (buttons, switches, keyboards, control sticks, etc.) must be able to operate either in direct real-life mode or in simulation (make-believe) mode. Similarly, all flight displays (gauges, television screens, lights, etc.) must be controllable in either of those modes, too.

With the spacecraft in simulator mode, another computer system somewhere else on board must maintain the mental mathematical model of the 'imaginary' practice spacecraft with its hypothetical practice problems.

This dual-mode operation scheme, while certainly an innovation for manned spaceflight, is feasible because of an advance in spaceship flight control systems characterized by the Space Shuttle's on-board computer quintet.

The technical term is 'fly-by-wire'; a technique used as a backup system in earlier manned spacecraft that has become the primary and only control system on the Space Shuttle. Essentially, all controller commands--engine firings, eleven pitch, gauge readings, whatever--come from the computer system, based on measurements taken throughout the vehicle, including from the crew's flight control switches, sticks and buttons.

Such a scheme, in which the computer system is programmed to select different control combinations depending on rapidly changing mission phases, was the only one judged capable of handling the intricate requirements of the Space Shuttle mission. But it required, in turn, a major advance in control theory and reliability of airborne computer systems.

So day by day, the Mars-bound astronauts would undergo landing simulations. Some could be quite normal, familiarizing them with the actual steps they hopefully would be following for the actual touchdown. Other runs would include difficulties that had to be detected, recognized and circumvented. This would be done with the aid of advice from Mission Control millions of miles away, where the data flow would be artificially delayed on tape to match the expected real delay connected with the distances at the time of the real landing.

Since the simulator control of the Mars spaceship could not tolerate any such delay, another on-board computer would have to act as the simulator control. This would require a high level of capability and special software programs. Perhaps the lander's computer system could double as the simulator computer for the mission module's simulations of Mars orbit insertion. In turn, the mission module's computers could double as the simulator computer when it came time for the lander to practice its own particular specialties.

Such simulations, involving the whole crew, could probably be scheduled as often as three times a week, for eight to ten hours each time. One astronaut would act as an on-board training official, while all the rest would be in actual training. That heavy load is in fact characteristic of astronaut training for major new missions. It was followed by the Apollo-11 moon-landing crew in the six months before their 1969 mission, by the first Space Shuttle Columbia crew over the same general time period before their 1981 mission, and by the Expedition-1 crew to the International Space Station in 1997-2000. And it was far from the hardest part of their preparation.

In fact, drawing on training experience for such analogous astronaut activities, the three full days of simulations per week would be just part of the crammed crew activities. They would also undergo special procedural and equipment briefings, probably for three half-days a week. They would be assigned to other specialized instruction on individual duties, probably in the form of videotapes or even motion holographs, for two other half-days. Their testing of the spacecraft for routine diagnostic functions would likely consume a couple of hours per crewman, twice a week. General space housekeeping, judging from Skylab, Mir, and International Space Station experience, would require about two hours per day per astronaut. This includes such duties as communications sessions, navigation updates, corrections to on-board documentation, and so forth.

Also from experience, the rest of each astronaut's day can be fairly well mapped out. Sleeping and personal hygiene takes nine to ten hours per day. Food preparation and mealtimes take two hours per day and maybe three on Sundays. Exercise to prevent the deterioration of muscles to be needed for walking around on Mars requires at least two hours a day on average, maybe with Sunday off. A major medical examination takes up one or two hours per crewmember per week. Personal time gets an occasional hour here or there and a big block set aside on Sunday. But if Skylab, Mir, and ISS are any example, that last item, together with sleeping time, is the most easily sacrificed when it comes to actually fitting everything together.

Somewhere in there, too, must be fit extensive scientific training for the Mars phase of the mission. Lander crewmembers would be expected to earn the equivalent of a correspondence course master's degree in geology; orbiter Mission Module crewmen, if any, would be doing the same but preparing primarily for visual and instrumental observations from orbit.

Each crewmember would also have collateral backup training in specialties of other crewmembers, in the event of somebody's being incapacitated. The Mars spaceship would take on all of the spirit and appearance of a flying university library in the week before final exams!